How to get a total...

Hi, I am really lost. I have spent an hour looking on the net and php.net for how to get total and how to do math with php. If anyone has a link to a site that show it or can example to me how to do it, that would be great. I have a small project where I am going to track invoices. But they want to be able to see how total amount of the invoice that I am tracking. I know this is got to be simple but it hard if I do have example of the commands to do it. Chuck Payne Magi Design and Support